Tony Gutierrez, AP
In this Jan. 31, 2012 photo, a book, foreground, written by Billy Frederick Allen sits on display as Allen addresses a question during an interview in his attorney's office in McKinney, Texas. Allen spent almost 30 years in prison before an appeals court overturned his convictions in two murders. Two years after winning his freedom, Allen is fighting one more battle: He wants the state to pay him more than $2 million he says he’s owed for wrongful imprisonment. (AP Photo/Tony Gutierrez)

Learning about the mistakes of the justice system is not easy. Americans have come to accept the bargain that freedom and constitutional protections offer, which is that some guilty people will go unpunished for lack of probable cause or because they could hire a defense team with the skills to persuade a jury. The other side of that coin, however, is disturbing. Some people who are innocent will go to prison, and perhaps even be executed.

This week, researchers at the University of Michigan Law School and the Center on Wrongful Convictions at Northwestern University School of Law released their analysis of a database of exonerations dating to 1989. As one of the professors involved notes, the analysis reveals some clear patterns about wrongful conviction. If there is good news in this study, it is that the justice system now has the opportunity to learn from its mistakes, and never before have those mistakes been so well documented.

For one thing, there are geographic patterns. Illinois, for instance, leads the nation in exonerations, followed by New York, Texas and California. What is not known is whether they rank high because they have a greater propensity for falsely convicting innocent people, or because they have taken seriously the need to review cases in light of new DNA technology.

The biggest patterns identified have to do with the types of mistakes associated with various crimes. For those convicted of murder, lies were the biggest hazard. Either they were falsely accused to begin with or suffered from perjured testimony on the witness stand. Fully two-thirds of the exonerations had this factor in common. For sexual assaults, 80 percent were wrongly convicted because of mistaken eyewitness identification.

Of the 873 exonerations studied carefully, nearly half were homicide convictions, and 101 of those involved death sentences. Some of the prisoners died before evidence could be found to exonerate them.

These were not minor mistakes. Half of those exonerated were in prison at least 10 years, and three quarters of them served at least five years. That's a huge chunk of life spent in an unsavory, unproductive place.

While the database is the most complete one of its kind assembled, it is only as good as the data available. Some counties reported no exonerations, which the researchers say couldn't be true. In addition, there may be many who were convicted of lesser crimes whose evidence has not been carefully reviewed, as well as others convicted of serious crimes who are in the same situation.

What this exoneration data cannot tell us is how many other wrongful convictions still stand. Nonetheless, better understanding of patterns reflected in the data can help the justice system to re-evaluate procedures. Certainly, DNA evidence should be a hedge against mistakes. It was the main factor in the release of nearly one-third of the homicide convicts and two-thirds of those exonerated of sexual assault.

But, as Time magazine noted, the exonerations often implicate many parties, from defense lawyers to prosecutors, judges, police officers and detectives. No one involved can afford to be cavalier where liberty and life itself are at stake.